Output value signals from radiometer or receiver channels are normalized to achieve a flat field response when creating a millimeter wave image. A normalizing factor is applied to the output value signals from each channel, and the normalizing factor accommodates drift in offset of the output value signals on a scan-by-scan basis. The normalizing factor is based on each channel observing a different mean scan brightness temperature from a portion of the scene than the mean brightness temperature of the entire scene. The normalizing factor is obtained by solving a system of simultaneous equations in which normalizing factors from all of the channels are related to one another, preferably by a consistency condition where the mean scan temperature of each channel is equal to an average of the intensities of those image pixels to which the output value signals from that channel contributes.
Legal claims defining the scope of protection, as filed with the USPTO.
1. A method used in millimeter wave imaging to flat field normalize output value signals from each of a plurality of channels which receive radiant energy emanating from points in a scene, comprising: determining a normalizing factor for the output value signals from each channel based on the output value signals obtained from a plurality of other channels; establishing the normalizing factor from information describing a drift in offset of the output value signals from each channel; and applying the normalizing factor to adjust the output value signals for each channel.
2. A method as defined in claim 1 , further comprising: establishing the normalizing factor on the basis that each channel observes a different mean scan brightness temperature of radiant energy scanned from a portion of the scene than the mean brightness temperature of the entire scene.
3. A method as defined in claim 2 , further comprising: establishing the normalizing factor for each channel from a system of simultaneous equations in which each equation is related to the others.
4. A method as defined in claim 3 , further comprising: setting up the system of simultaneous equations in which the normalizing factor for each channel is related to the normalizing factors for all of the other channels.
5. A method as defined in claim 3 , further comprising: establishing the normalizing factor for each channel from an equation substantially the same as Equation (15).
6. A method as defined in claim 3 , further comprising: establishing the normalizing factor for each channel from the system of simultaneous equations in which each equation is related to the others by the mean scan brightness temperatures encountered by each channel.
7. A method as defined in claim 6 , further comprising: solving the system of simultaneous equations to obtain values for the normalizing factors for each channel; applying the normalizing factors to the output value signals from each channel to create normalized output signals; and composing an image from the normalized signals.
8. A method as defined in claim 7 , further comprising: calculating the normalizing factor for each channel with each scan of radiant energy from the entire scene into the plurality of channels.
9. A method as defined in claim 7 , further comprising: solving the system of simultaneous equations and applying the normalizing factors with each scan of radiant energy from the entire scene used to compose the image.
10. A method as defined in claim 7 , further comprising: composing the image with a plurality of pixels, each pixel corresponding to a point in the scene; scanning the radiant energy emanating from each point in the scene into a plurality of different channels; and composing an intensity of each pixel of the image from the normalized output signals from each channel into which the radiant energy emanating from the corresponding point in the scene is scanned.
11. A method as defined in claim 10 , further comprising: calculating the normalizing factors based on a consistency condition that the mean scan temperature encountered by each channel is equal to an average of the intensities of those pixels of the image to which the output value signals from that channel contributes.
12. A method as defined in claim 11 , further comprising: calculating the average of pixel intensities based on an equation defining the average pixel intensities in terms of the mean scan temperatures of each channel.
13. A method as defined in claim 10 , further comprising: calculating the normalizing factors for composing the intensity of the pixels of each image with each scan of radiant energy from the entire scene to form that image.
14. A method as defined in claim 10 , further comprising: composing the intensity of each pixel by adding the normalized output signals from each channel which contribute to the intensity of each pixel.
15. A method as defined in claim 10 , further comprising: establishing the normalizing factor by which to create the normalized output signals from the output value signals from each channel from an equation substantially the same as Equation (15); and solving Equation (15).
16. A method as defined in claim 15 , further comprising: solving Equation (15) by expressing Equation (15) in terms of a singular matrix A and a vector b.
17. A method as defined in claim 16 , further comprising: setting one of the normalizing factors to an arbitrary value representative of a relationship that the mean scan brightness temperatures of each channel is relative.
18. A method as defined in claim 17 , further comprising: relying on the setting of one of the normalizing factors to an arbitrary value of zero to create a full rank non-square matrix A′ from the singular square matrix A; establishing that the values of the remaining normalizing factors are independent; and computing a normalizing factor for each channel from a solution of the system of equations based on the matrix A′.
19. A method as defined in claim 18 , further comprising: computing the normalizing factor for each channel using an inversion of the matrix A′.
20. A method as defined in claim 19 , further comprising: computing the normalizing factor independently of establishing the output value signals from each channel in response to each scan of radiant energy from the entire scene used to create an image.
21. A method as defined in claim 15 , further comprising: expressing Equation (15) as a matrix equation; solving the matrix equation; and establishing the normalizing factor applicable to each channel by a value related to the solution to the matrix equation.
22. A method as defined in claim 21 , further comprising: solving the matrix equation independent of each scan of radiant energy from the entire scene used to compose the image.
23. A method as defined in claim 22 , further comprising: using the solution of the matrix equation to establish the same normalizing factor for each channel for each of a plurality of subsequent scans of radiant energy from the entire scene used to compose a corresponding plurality of images of the scene.
24. A method as defined in claim 21 , further comprising: computing a pseudo-inverse of the matrix; computing a vector based on the output value signals from the channels; and multiplying the pseudo inverse of the matrix and the vector to obtain the normalizing factors for each channel.
25. A method as defined in claim 24 , further comprising: computing the intensity of each pixel in the image by applying the normalizing factor to each output value signal from each channel which contributes to the intensity of each pixel.
26. A method as defined in claim 15 , further comprising: obtaining a magnitude of a scene-independent baseline signal component of each output value signal from each channel at each position of a movable scanning element which directs radiant energy from the scene to each channel; and subtracting the magnitude of the baseline signal at each position of the movable scanning element from each output value signal of each channel derived at corresponding positions of the movable scanning element to create baseline-compensated output value signals.
27. A method as defined in claim 26 , further comprising: measuring the magnitude of the baseline signal at each position from which radiant energy is directed into the channel from a scene of uniform brightness.
28. A method as defined in claim 27 , further comprising: applying the normalizing factor applicable to each output value signal to the corresponding baseline-compensated output value signal to create normalized baseline-compensated output value signals; and composing the intensity of each pixel from the normalized baseline-compensated output value signals applicable to each pixel.
29. A method as defined in claim 28 , further comprising: weighting each baseline-compensated output value signal by a different amount prior to adding the baseline-compensated output value signals from each of the plurality of channels to compose the intensity of each pixel.
30. A method as defined in claim 29 , further comprising: weighting each baseline-compensated output value signal by a predetermined weighting factor related to an amount of noise response from each output value signal from each channel.
31. A method as defined in claim 30 , further comprising: using as the weighting factor the reciprocal of the standard deviation of the amount of noise in each output value signal from each channel.
32. A method as defined in claim 31 , further comprising: measuring a magnitude of noise from each channel into which radiant energy is scanned from a scene of uniform brightness; and computing the standard deviation of the output value signals from each channel based on the measured magnitude of noise.
33. A millimeter wave camera implementing the method defined in claim 31 , comprising: a signal processor for performing actions and calculations to subtract the baseline signal component from the output value signals to establish the baseline-compensated output signals, to weight the value of each baseline-compensated output signal, to normalize the baseline-compensated weighted output signals, and to compose the intensity of each pixel of the image by adding the values of the normalized, baseline-compensated and weighted signals; the signal processor performing the actions and calculations in response to the output value signals from each channel from scanning a scene having a non-uniform brightness temperature, to gain information describing an amplification capability of each channel, and to deviations in response characteristics of each channel in response to scanning a scene of uniform brightness temperature.
34. A method as defined in claim 1 , further comprising: establishing the normalizing factor from information also defining the individual gain of each channel.
35. A method as defined in claim 34 , further comprising: measuring the individual gain of each channel in response to scanning radiant energy from two scenes of different and known uniform brightness temperatures into that channel; and using the measured gain of each channel to establish the normalizing factor.
36. A method as defined in claim 34 , further comprising: calculating the normalizing factor from information defining the individual gain and offset characteristics of each of the channels.
37. A method as defined in claim 1 used in passive millimeter wave imaging.
Cooperative Patent Classification codes for this invention. Click any code to explore related patents in that topic.
January 31, 2003
April 12, 2005
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